Injection devices are routinely used in the medical field to deliver a measured dose of medicine to a user. Due to their user-friendly design, they can be safely used by patients for self-administration, although in some circumstances they may be used by trained medical personnel.
A typical injection device comprises a relatively large number of parts. For example, FIGS. 1 to 4 illustrate schematically an OWEN MUMFORD AUTOPEN® device suitable for use with prefilled medicine cartridges, i.e. a device type commonly referred to as a “pen injector”. A related description is provided in WO/2007/063342. FIG. 1 shows a side profile of the pen injector. FIG. 2 shows the pen injector in a fully reset configuration. FIG. 3 shows the pen injector in a fully discharged configuration. FIG. 4 provides an expanded view of a central section of the device.
With reference to FIGS. 1 to 4, the known pen injector comprises a housing 1 to which a receptacle 2 is connected. The receptacle 2 is arranged to receive the medicine cartridge 3. The pen injector has a cover 6 for protecting the receptacle 2 when not in use. The housing 1 is provided with a trigger 7 for actuating the device, and a dose knob 8 for selecting a dose. The housing 1 contains a torsion spring 9. The torsion spring 9 is located coaxially within the housing 1 and is arranged to provide a drive force for ejecting medicine from the cartridge 3.
A generally cylindrical ratchet drive shaft 10 extends through the centre of the torsion spring 9. An enlarged end portion 10a of the shaft 10, located at a proximal end, and has three sprung legs formed around its periphery, the legs being spaced equally around the shaft 10. At the outermost end of each leg, a tooth is provided. These teeth engage with a rack (not shown) formed around the inner surface of a drive gear 11, which sits within the housing 1 at a fixed axial position. The drive gear 11 has a second toothed rack 11a formed around a lower outer surface portion. The rack 11a sits within a correspondingly sized rack 12a formed on an inner surface of a retaining ring 12. The ring 12 is formed integrally with the trigger 7, with the trigger 7 being slidably mounted within a slot formed in the housing 1. A spring 13 urges the trigger 7 in a distal direction, maintaining the racks 11a and 12a in locking engagement in the absence of a user applied force.
A leadscrew 14 has a screw thread formed along the length of its outer surface. The leadscrew 14 is located within the ratchet drive shaft 10, and engages a complimentary screw thread formed on the inner surface of the drive gear 11. An end portion of the leadscrew projects from the ratchet drive shaft 10 and has a leadscrew cap 15 secured thereto. The cap 15 is rotationally attached to the leadscrew 14, such that it can be rotated relative to the leadscrew 14. The leadscrew 14 has axial guide tracks 16 extending along its outer surface and which engage with a locking bush 17 via splines 18 on the locking bush 17. The locking bush 17 is slidably held within a mid-body compartment 19 which itself is secured to the end of the housing 1 via a pair of complimentary screw threads. The locking bush 17 has a serrated edge 20 running along an end for engaging with mating features of the housing 1.
Consider now the operation of the pen injector. A user sets a dose by rotating the dose knob 8 in a clockwise direction. As the dose knob 8 is rotated, the distal end of the torsion spring 9 rotates with it, accumulating energy in the spring. Engagement of sprung fingers (not shown) at a distal end of the ratchet drive shaft 10 with a rack formed on the inner surface of the dose selector 8 also causes the ratchet drive shaft 10 to rotate. At a proximal end of the ratchet drive shaft 10, the teeth of the sprung legs “click” around the rack (not shown) formed around the inner surface of the drive gear. The engagement of the teeth with the rack at the proximal end of the ratchet drive shaft 10 prevents the spring 9 unwinding after each click. Each click corresponds to a predefined angular rotation of the spring and therefore to a predefined ejection dose. It will be readily appreciated that, during the dose setting action, the drive gear 11 is not rotated so no axial movement of the leadscrew 14 is induced. No medication is therefore ejected from the cartridge during the dose setting operation (or indeed air introduced due to back filling).
Once a dosage is set, the user can apply a force to the trigger 7 in the proximal direction. This disengages the rack 12a of the trigger 7 from the rack 11a of the drive gear 11. This frees the drive gear 11 and the torsion spring 9 to rotate. As the drive gear 11 rotates about the leadscrew 14, the leadscrew 14 is driven through the drive gear 11 causing the leadscrew cap 15 to push the bung of the cartridge 3 through the cartridge body, expelling medication from the cartridge 3 through an attached syringe.
The known pen injector is able to deliver several doses from the same medication filled cartridge 3. During delivery of each successive dose, the leadscrew 14 is advanced further forward into the medication filled cartridge 3. This continues until an enlarged end 21 of the leadscrew 14 engages with a distal end of the drive gear 11, at which point the leadscrew 14 has reached the end of its travel. FIG. 3 shows the known pen injector in this position.
During the injection process, the leadscrew 14 must be prevented from rotating relative to the housing 1. However, following the removal of the spent cartridge, it must be possible to push the leadscrew 14 back into the housing 1 to a starting position, this operation requiring rotation of the leadscrew within the housing. This is achieved by means of the locking bush 17.
Following removal of the receptacle from the housing, a spring 23 urges the locking bush 17 in the proximal direction relative to the housing, disengaging the locking bush from the housing and allowing it to rotate relative thereto. Once a new cartridge has been loaded into the receptacle 2, the user attaches the receptacle 3 onto the end of the housing 1. On coupling of the receptacle 2 to the housing 1, the leadscrew cap 15 abuts the bung in the end of the cartridge 3. The user pushes the receptacle 2 towards the housing 1, causing the leadscrew 14 to rotate within the drive gear 11 and move back into the housing 1. When the screw thread on the receptacle 2 engages with that on the housing 1, the user screws the two parts together, with the leadscrew 14 continuing to rotate and move into the housing 1 during this process.
On final coupling with the receptacle 2, the receptacle 2 and cartridge 3 engage with a cartridge compression cup 22, which compresses the spring 23 and transmits the loading onto the locking bush 17. The locking bush 11 re-engages with the housing, preventing the locking bush 17 from further rotation and also locking the leadscrew against rotation by means of the engagement of the splines 18 with the axial guide tracks 16. This ensures that the leadscrew 14 can move forward when subjected to rotation following release and rotation of the drive gear 11.
Medication filled cartridges are generally tubular in shape, although, due to structural considerations, have a slightly tapered portion towards the needle receiving end. If the bung is allowed to be pushed into this tapered end region there is a danger a) that the cartridge 3 may fracture and b) that an unreliable dose will be delivered (as the dosing mechanism assumes a uniform cartridge cross-section). Ideally, the device is configured such that the cap 15 cannot move beyond a point at which the bung is about to enter the tapered end portion of the cartridge 3. This is achieved by configuring the device such that the enlarged end 21 of the leadscrew 14 engages with a distal end of the drive gear 11 to stop the leadscrew, and therefore the bung, at the correct position relative to the cartridge. Of course, the configuration must take account of manufacturing tolerances, meaning that, in practice, the end 21 must be stopped at some distance distal from the “theoretical” stopping point, i.e. the device must be designed to provide a “buffer zone”.
The buffer zone that must be incorporated into the device will depend upon the number of connected components. In the case of the device of FIGS. 1 to 4, the tolerance with which the stopping point of the end 21 can be defined depends upon the sum of the individual tolerances of the housing 1, receptacle 2, drive gear 11, mid-body compartment 19, and locking bush 17. This might be as much as 0.6 mm, meaning that on average the end of the bung will stop 0.6 mm distal from the desired stopping point in the cartridge. As a result, on average, a significant amount of medicine will remain in the cartridge 3 after a final injection from the cartridge. This has significant cost implications.